How to Splice a Porch Into an Existing Roof

Match the porch roof to the existing roof, with the same width or span and pitch or slope of the sides. Determine the pitch of the existing roof by putting a level under a rafter or truss top chord, touching that board at one end. Measure 12 inches along the level with a tape measure and measure the distance between the top of the level and the bottom of the rafter; if that is 6 inches, the roof is a 6/12, sloping 6 inches per foot.

Order prefabricated trusses to match the original roof in width, pitch and overhang on the side eaves. Get one truss for every 24 inches of porch length, plus one extra to fasten to the original roof. Duplicate the existing framing if possible, but it is not necessary to match it exactly if the span, pitch and overhang are identical; you can combine porch trusses with conventional rafter framing.

Strip the gable end of the existing roof down to the end rafters or trusses. Remove all siding, sheathing, shingles and other material until the last rafter on the end is fully exposed. Check the height of the porch walls and the house walls; add boards or shim the porch tops if necessary to make the heights identical so porch and house framing will line up.

Install a 2-by-6-inch ledger board on the house gable with the top at the height of the porch tops; this should align with a rim joist on the house roof. Fasten the ledger to the joist with 1/2-inch lag bolts driven with a ratchet wrench through holes drilled with a power drill and 3/8-inch bit. Use a level to ensure the ledger is level.

Mark the location for trusses. Draw a line across the porch wall tops 1.5 inches from where they touch the house roof for a first porch truss. Measure 23.25 inches from the house roof and mark a line for the outside of the second truss. Use that line as a base to mark trusses at 24-inch intervals the length of the porch.

Set the first truss in place and nail it to its counterpart on the house roof with 16d framing nails and a hammer. Fasten the bottom chord to a horizontal chord or joist on the house roof. Nail the sloped sides to the angled rafters or truss chords on the house roof. Nail the ends of the new truss to the porch wall caps on both sides. Plumb this truss with a level; use tapered wood shims if necessary to get it plumb.

Add other trusses one at a time the length of the porch. Fasten them to the porch wall caps. Replicate on the porch roof any diagonal cross-bracing used to tie trusses or rafters together in the house..

Cover the framed roof with oriented strand board nailed to the trusses with 8d galvanized nails. Staple roofing paper with a construction stapler over the OSB, ensuring one layer overlaps the peak. Put on shingles to match the original roof, starting at the splice point. Cut every other row of old shingles back by one tab so new and old shingles will overlap at the splice.